On 27.04.2018 15:36, Robin Murphy wrote:

Hi Thierry,

On 27/04/18 11:02, Thierry Reding wrote:

quoted

On Mon, Apr 09, 2018 at 11:07:22PM +0300, Dmitry Osipenko wrote:

quoted

Currently GART writes one page entry at a time. More optimal would be to
aggregate the writes and flush BUS buffer in the end, this gives map/unmap
10-40% (depending on size of mapping) performance boost compared to a
flushing after each entry update.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
  drivers/iommu/tegra-gart.c | 63 +++++++++++++++++++++++++++++++++++-----------
  1 file changed, 48 insertions(+), 15 deletions(-)

diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
index 4a0607669d34..9f59f5f17661 100644
--- a/drivers/iommu/tegra-gart.c
+++ b/drivers/iommu/tegra-gart.c

@@ -36,7 +36,7 @@

  #define GART_APERTURE_SIZE    SZ_32M
    /* bitmap of the page sizes currently supported */
-#define GART_IOMMU_PGSIZES    (SZ_4K)
+#define GART_IOMMU_PGSIZES    GENMASK(24, 12)

That doesn't look right. The GART really only supports 4 KiB pages. You
seem to be "emulating" more page sizes here in order to improve mapping
performance. That seems wrong to me. I'm wondering if this couldn't be
improved by a similar factor by simply moving the flushing into an
implementation of ->iotlb_sync().

That said, it seems like ->iotlb_sync() is only used for unmapping, but
I don't see a reason why iommu_map() wouldn't need to call it as well
after going through several calls to ->map(). It seems to me like a
driver that implements ->iotlb_sync() would want to use it to optimize
for both the mapping and unmapping cases.

Joerg, I've gone over the git log and header files and I see no mention
of why the TLB flush interface isn't used for mapping. Do you recall any
special reasons why the same shouldn't be applied for mapping? Would you
accept any patches doing this?

In general, requiring TLB maintenance when transitioning from an invalid entry
to a valid one tends to be the exception rather than the norm, and I think we
ended up at the consensus that it wasn't worth the complication of trying to
cater for this in the generic iotlb API.

To be fair, on simple hardware which doesn't implement multiple page sizes with
associated walk depth/TLB pressure benefits for larger ones, there's no need for
the IOMMU API (and/or the owner of the domain) to try harder to use them, so
handling "compound" page sizes within the driver is a more reasonable thing to
do. There is already some precedent for this in other drivers (e.g. mtk_iommu_v1).

Probably the best variant would be to give an explicit control over syncing to a
user of the IOMMU API, like for example device driver may perform multiple
mappings / unmappings and then sync/flush in the end. I'm not sure that it's
really worth the hassle to shuffle the API right now, maybe we can implement it
later if needed. Joerg, do you have objections to a 'compound page' approach?
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